Learn Turkish: Elmalar tatlı ama ben ekşileri seviyorum. - The apples are sweet, but I like the sour ones.

This is an example of a substantivized adjective, where an adjective takes on the role of a noun. The word ekşi means sour. By simply adding the plural suffix -ler, we get ekşiler (sour ones). You do not need a separate word for "ones" in Turkish!

Why is there an -i at the very end of ekşileri?

That is the definite accusative case suffix. The verb sevmek (to like/love) requires its specific direct object to be in the accusative case. Because you are talking about liking a specific group (the sour ones, contrasted with the sweet apples), ekşiler takes the accusative ending to become ekşileri.

Why doesn't tatlı have a plural suffix, since it describes elmalar (apples)?

In Turkish, when an adjective is used to describe what a subject is, it usually does not take a plural suffix if the subject is already plural. So, elmalar tatlı is perfectly fine. If you said tatlılar, you would be substantivizing the adjective, which would mean "the sweet ones" or "the sweets/desserts".

Since seviyorum already means "I like," why is the pronoun ben used in this sentence?